power: constify of_device_id array

of_device_id is always used as const.
(See driver.of_match_table and open firmware functions)

Signed-off-by: Fabian Frederick <fabf@skynet.be>

[for vexpress]
Acked-by: Sudeep Holla <sudeep.holla@arm.com>

Signed-off-by: Sebastian Reichel <sre@kernel.org>